1. Field of the Invention
The present invention relates to a MOS image sensor IC used for capturing and electrically transmitting image information in an apparatus such as a facsimile, an image scanner, and a digital camera.
2. Description of the Related Art
A circuit diagram of a conventional MOS image sensor is shown in FIG. 4 as an example. In a sensor circuit 10 of the MOS image sensor, a photodiode 12 with a PN junction is connected to a reset transistor 11 serving as a switching element for resetting the photodiode 12 to an appropriate voltage, and to an amplifier 13 for amplifying photoelectric charges accumulated in the photodiode 12.
Continuous capture of optical information can be carried out through operations including: reset operation in which the reset transistor 11 is turned on to hold the photodiode 12 at a reset voltage; accumulation operation in which the reset transistor 11 is turned off to accumulate photoelectric charges in the photodiode 12 for a certain period of time; and read operation in which the amplifier circuit 13 is turned on to amplify the photoelectric charges accumulated in the photodiode 12 to read.
A holding circuit 20, including two switching transistors (22A and 22B) and a capacitive element 21, permits temporary storage of a signal which has been amplified after the read operation.
The switching transistor 22A is turned on during the read operation to store the signal into a holding capacitor 21 as an electric charge through the amplifier 13, and the switching transistor 22A is turned off. The switching transistor 22B is then turned on after an arbitrary holding time to permit the signal reading from the holding capacitor 21.
A series of reset operation, accumulation, and read operation are collectively performed on a plurality of photodiodes to permit arbitrary and independent signal reading from the holding circuit.
Photoelectric conversion, whose characteristic is one of the most important characteristic, is performed in accordance with the intensity of incident light to the photodiode 12.
For improvement of the photoelectric conversion characteristic, JP 2004-312039 A (FIG. 24), for example, discloses a photoelectric conversion element in which occurrence of a defect is suppressed in a semiconductor region in which a depletion layer is formed inside the photodiode 12.
The image sensor IC having a plurality of pixels formed in a single chip, however, shows a problem in which variation in photoelectric conversion characteristic of a plurality of photodiodes 12, which constitutes the plurality of pixels, occurs due to the change in the intensity of the incident light caused by variation in thickness of a protective film formed on an upper surface of the photodiodes 12.
Countermeasures for the problem have been proposed: for example, planarization after the formation of the protective film and the formation of a second protective film so as to further make the film thickness uniform. There still exist problems that, for example, the number of processes is increased to thereby increase manufacturing costs, and that the sufficient uniformity in film thickness cannot be obtained.